Loop assembly



May 4, 1943.

A. F. BISCHOFF 2,318,361

LOOP ASSEMBLY Filed Sept. 1:; 1940 Inventor Alfred FT Bischoff,

by Wa2 His tor-ney.

Patented May 4, 1943 LOOP ASSEMBLY Alfred F. Bischoff, General Electric New York Milford, Conn., assignor to Company, a corporation of Application September 13, 1940, Serial No. 356,681

7 Claims.

My invention relates to inductance coils and more particularly to coils suitable for use as loop antennas for radio receivers.

An object of my invention is to provide an improved form of industance coil whose inductance may be easily adjusted and whose manufacture is simple and cheap.

It is a further object of my invention to provide such an inductance coil which is especially useful as a loop antenna for small radio receivers, the inductance of the coil being readily adjusted to provide the proper tuning of the receiver during its manufacture.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates an embodiment of my invention connected in a conventional schematic radio receiver circuit; and Fig. 2 illustrates a radio receiver embodying my invention.

In Fig. 1 a loop antenna I is connected in shunt to an adjustable tuning condenser I I to form the first radio frequency circuit in a conventional radio receiver. Waves to which this circuit is tuned appear across the condenser II and, through the action of a conventional oscillator I2, a converter discharge device I3, and an intermediate frequency and audio amplifier I4 are converted into audio signal current. A loud speaker I5, energized by these audio signal currents, reproduces sound modulated on the carrier wave picked up by the antenna I0. A variable condenser l6, which controls the frequency of the oscillator I2, has a mechanical control which is also common to the condenser II, as in conventional superheterodyne receivers.

The loop antenna I9 comprises a plane piece of fiber board II which is coated with wax to have good insulating qualities and which has slots I8 spaced regularly along the periphery thereof. A first coil of wire I9 is wound back and forth through these slots I 8 along the periphery of the coil form I'I. Terminal posts 20 and 2| are attached respectively to the ends of the coil I9 and are fastened to and supported on the coil form II. Connections extend from these terminal posts 20 and 2I respectively to opposite terminals of the condenser II to form the tuned loop antenna circuit previously referred to.

Outside of the coil I9 there is a second coil 22 of wire which is also wound through the slots I8 around the periphery of the coil form II. A pair of terminal posts 23 and 24 are connected respectively to the ends of this second coil 22 and are attached to a support; from the coil form II. If an external antenna be available, connection may be made from it to terminal post 23. By condenser action between the coils 22 and I9 signals may be transferred from this external antenna to the coil I9 which is tuned by the condenser II. If, in addition, a ground is readily available as well as an external antenna, it may be connected to the terminal post; 24, so that Waves from the external antenna and ground are transferred to the coil is through transformer action, both condenser action and transformer I action being effective to transfer power in phase.

In a superheterodyne receiver it is necessary that the tuned circuit including the condenser II and the tuned circuit of the oscillator I2 including the condenser It be always tuned to frequencies which differ by a fixed amount to maintain high sensitivity. This fixed amount is equal tothe intermediate frequency and such adjustment is necessary for proper operation of the receiver. My improved inductance coil II] includes means for adjusting the frequency of the tuned circuit formed by the coil III and the inductance II so that the receiver may have high sensitivity at any frequency to which it is tuned.

In the intermediate portion of the coil form I! within coil I9 at two spaced points there are fastened posts 25 and 26 which support a flexible conducting wire loop 21. The conducting wire of the loop 21 is of such material that it may be readily bent into any desired shape by manipulation in the hands of the operator and is sufficiently retentive and of sufficiently low resilience or relative inelasticity to maintain its shape in any form to which it is bent. This loop 21 is, of course, in inductive relation to coil I9 and comprises a closed circuited path affecting the inductance of the loop antenna I9. It thus serves as a very convenient means for adjusting the inductance of the coil I9.

If the loop 21 be bent so as to enclose the maximum area, as illustrated by the dotted line figure 28, the inductance of the coil I9 is minimum. If the three sides of the loop 21 be bent together and across each other Without making contact, as illustrated by the dotted line figure 29, so that the area 30 enclosed by the inwardly bent three sides of the loop 21 is equal to the sum of the areas SI and 32 enclosed by the remaining portions of the loop 21, the effective inductance of the coil I9 is maximum. At any intermediate position between these two extremes such as the position of conductor 21 shown in full lines in Fig. l, the effective inductance of the coil I9 is of an intermediate value.

For purposes of factory adjustment it is convenient to manufacture the loop antenna III with the loop 21 in an intermediate position, as shown in full line on the drawing. The effective inductance of the coil I9 may then be either increased or decreased by the operator during the test of the receiver in which the loop is used simply by suitably altering the shape of the loop with her hands, or With instruments if desired, to make the loop 21 enclose respectively less or more effective area. The tuning of the input circuit including the coil I9 and the condenser II may be thus conveniently adjusted to operate at a frequency different from the frequency of the oscillator I2 by the amount of the intermediate frequency throughout the tuning range of the receiver. In this way the adjustment of tracking, or alignment, of the tuned circuit and oscillator may be readily effected.

In Fig. 2 a small radio receiver, drawn in section, illustrates the physical relation of the loop antenna Iii to the remaining parts of the receiver. This small radio receiver includes a metal chassis 33 mounted inside of a cabinet 34 with tuning and volume control 35 on the exterior thereof. The loud speaker I and the electron discharge device I3 are mounted on and supported from the metal chassis 33. The cabinent 34 has a back cover 36 to the inner surface of which the loop antenna I0 is fastened and supported.

If the loop antenna I0 were used in free space, the ratio of its inductive reactance to its effective resistance would be quite high. This ratio, however, would change upon adjustment of the loop El by a percentage approximately twice as great as any change made in the inductance by such adjustment.

' Since the loop antenna I0 is supported near the metal chassis 33 and other metal parts of the receiver, the ratio of inductive reactance to effective resistance or, in other words,the effective damping, of the loop antenna I9 is about half as great as in free space. Under such a condition the effective damping of the coil I9 changes by only a small fraction of any percentage change made in the inductance of the coil I9 by adjustment of the loop 2?. It has been found that such a small change in the effective damping of the coil I9 does not produce noticeable differences in sensitivity between various receivers which have their loops 2I differently adjusted.

The above described loop antenna It with its adjusting loop 27 is most desirably used in small radio receivers where it is necessary to mount the antenna II] in close proximity to the chassis 33. However, it is also advantageous in larger receivers, where portability is desired or where no external antenna is available, to reduce the change in effective damping of the coil I9 caused by adjustment of the loop 21, and it is desirable to have the loop antenna I9 mounted in close proximity to the chassis 33.

In the construction of a particular loop antenna according to my invention the coil I9 was formed of 27%.; turns and the coil 22 of two turns. The loop 21 was formed of No. 16 wire having a diameter of 1.628 millimeters: The

wire in the loop 21 was slightly less than 12 inches long, and formed roughly a rectangle in its intermediate position about 4 inches long and 1 inches wide. It was found that the ratio of inductive reactance to efiective resistance of the coil I9 at 600 kilocycles was about 130. Upon placing the antenna I0 near the metal chassis of the receiver for which it was designed, the ratio of inductive reactance to effective resistance of the coil I9 wasabout 72. Adjustment of the loop 2? to provide a variation in total inductance of the coil I9 of about 10 per cent then produced only about 4 per cent change in the ratio of inductive reactance to effective resistance of the coil I9. This small change in ratio is not noticeable inthe receiver sensitivity.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed and I aim in the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a loop antenna comprising a coil, and a closed circuited turn in the form of a relatively inelastic conductor positioned in inductive relation with said coil, said conductor having sufficient flexibility that it may be readily manipulated by the fingers of the operator into different shapes thereby to vary the inductance of said coil and having sufficient retentivity and low resilience substantially to retain any shape into which it is manipulated, one of said shapes being that of a single loop to make the inductance of said antenna a minimum and another of said shapes being that of a plurality of loops to make the inductance of said antenna a maxiinum.

2. In combination, a radio receiver having ametal chassis and a variable condenser mounted thereon, a loop antenna comprising a coil mounted near said chassis, said coil and con-"- denser forming a variable tuning circuit for said radio receiver, said circuit being damped by reason of the proximity of said loop to said chassis, and a closed circuited relatively inelastic conductor positioned in inductive relation with said coil for Varying the inductance thereof, said conductor being flexible and of low resilience substantially to retain any shape into which it is manipulated by the operator and to retain any shape into which it is manipulated when re-' leased by the operator, whereby manipulation of said conductor is effective to vary the area enclosed thereby to vary the inductance of said coil to adjust the resonant frequency of said tuning circuit while producing a relatively small change in damping in said circuit due to the damping produced by said chassis.

3. A loop antenna comprising a plane coil form, a coil supported adjacent the edges of said form, and a closed circuited conductor supported on said form within said coil in inductive relation therewith, said conductor having su'ffi cient flexibility that it may be readily manipulated to different shapes thereby to vary the inductance of said coil, and having sufficiently low resilience substantially to retain any shape into which it is manipulated when released by the operator, one of said shapes being a single loop to make the inductancev of said antenna a minimum and another of said shapes being a plurality of loops to make the inductance of said antenna a maximum.

4. In combination, an inductance coil, and a closed circuited turn of substantially inelastic conductor positioned in inductive relation with said coil, said conductor having sufficient flexibility that it may be readily manipulated by the operator into diii'erent shapes to vary the area enclosed by said conductor thereby to vary the inductive relation between said coil and turn and to vary the inductance of said coil, said conductor having sufiicient retentivity and low resilience substantially to retain any shape into which it is manipulated when released by the operator.

5. In combination, an inductance coil utilized in proximity to a conducting body, said body having sufficient size substantially to reduce the ratio of inductance to resistance of said coil, a closed circuited turn of substantially inelastic conductor positioned in inductive relation with said coil, said conductor being suiiiciently flexible to be readily manipulated by the fingers of the operator to vary the area enclosed by said conductor to permit adjustment of the inductance of said coil and having sufficient retentivity to retain any shape to which it is manipulated when released by the fingers whereby the inductance of said coil may be varied over a substantial range by variation of the shape of said turn while variation of the ratio of inductance to resistance produced by said turn is reduced by the proximity of said conducting body.

6. In combination, an inductance coil, and a closed circuited turn of substantially inelastic conductor positioned in inductive relation with said coil, said conductor having suilicient flexibility that it may be readily manipulated by the operator into different shapes to vary the area enclosed by said conductor thereby to vary the inductance of said coil and having sufficient substantially and low resilience relatively to retain any shape into which it is manipulated when released by the operator, and said turn enclosing an area substantially unaffected by temperature variations to which said coil may be subjected in normal operation.

'7. A loop antenna comprising a plane coil form, a coil supported adjacent the edges of said form, and a closed circuited conductor supported on said form within said coil in inductive relation therewith, said coil and said conductor lying substantially in the plane of said form, said conductor having sufiicient flexibility that it may be readily manipulated to different shapes substantially in the plane of said form in order to vary the inductance of said coil and having sufiiciently low resilience substantially to retain any shape into which it is manipulated, one of said shapes being a single loop to make the inductance of said antenna a minimum and another of said shapes being a plurality of said loops to make the inductance of said antenna a maximum.

ALFRED F. BISCHOFF. 

